In a network, data passes through network nodes. The network nodes allow data to be routed to their destinations. Such network nodes often include a plurality of line cards and one or more control cards. The line cards are coupled to a plurality of lines, while the control cards control the line cards.
Network node clocks typically have a central oscillator that can serve several functions, including, but not limited to being a local oscillator for the node's Synchronous Equipment Timing Source (“SETS”) Phase-Locked Loop (“PLL”) during locked operation, providing the SETS PLL with frequency holdover in the event of all references being lost and/or proving the node with time holdover in the event of all references being lost.
Historically, a network node which has holdover requirements in the network is constrained by the need to keep its frequency within certain limits when in holdover mode. With newer applications, such as mobile networks, requiring time delivery from the network, there are new and much more stringent holdover requirements evolving that require a node to keep time within set limits. For example, a node which loses all of its frequency references may need to keep time to within a few microseconds for a day or more.
The choice of oscillator for these types of applications is a tradeoff between cost and performance and in particular, frequency stability performance. The oscillator is typically soldered down on a central function card such that it may not be replaceable. The choice of oscillator and thus the tradeoff is determined at the time the central function card is designed and then provisioned.
This can be a problem if different applications require different cost/performance tradeoffs for their oscillator. Because the oscillator is soldered down and its output is directly connected to the SETS PLL input on the central function card, the oscillator alone cannot be field upgraded.
If application requirements of the network node change, the central function card needs to have a design upgrade and then be swapped out in the field, which can incur significant development costs for the equipment provider and disruption to the network operator using the node.
Furthermore, it is possible that a design upgrade to the central function card circuitry in order to improve oscillator performance, is not possible due to the physical size increase of a higher-stability oscillator.
Currently, in order to meet evolving holdover requirements of a network node clock's central oscillator, a user must perform a design upgrade of the central function card, or entire node if that central resource is not field-upgradeable with an appropriate oscillator, and then swap out the circuit card assemblies (“CCAs”) or the entire node in the field.